Solution Gas-Oil Ratio Correlations

Overview

The solution gas-oil ratio ($R_s$) is defined as the volume of gas dissolved in oil at reservoir conditions, expressed as standard cubic feet of gas per stock tank barrel of oil (scf/STB). It is one of the most important PVT properties for reservoir engineering calculations.

Where:

• $V_{gas}^{SC}$ = Volume of dissolved gas at standard conditions
• $V_{oil}^{SC}$ = Volume of oil at standard conditions

$R_s$ varies with pressure:

• Below bubble point ($P < P_b$): $R_s$ increases with increasing pressure as more gas dissolves
• At bubble point ($P = P_b$): $R_s$ reaches its maximum value ($R_{sb}$)
• Above bubble point ($P > P_b$): $R_s$ remains constant (all gas is dissolved)

Theory

Physical Basis

Gas solubility in crude oil depends on:

1. Pressure: Higher pressure forces more gas into solution
2. Temperature: Higher temperature decreases gas solubility (opposite of water)
3. Gas composition: Lighter gases (methane) dissolve more readily
4. Oil composition: Lighter oils dissolve more gas

Correlation Approach

Since the 1940s, researchers have developed empirical correlations relating $R_s$ to readily available field data:

These correlations are region-specific, developed from PVT data collected in particular oil-producing areas. Selecting the appropriate correlation for your reservoir is critical for accuracy.

Correlation Equations

Standing Correlation (1947/1981)

Developed from California crude oil data. One of the earliest and most widely used correlations.

Where:

Applicability Range:

Vasquez-Beggs Correlation (1980)

Developed from a worldwide database of over 6,000 data points. Uses corrected gas gravity for separator conditions.

For $\gamma_{API} \leq 30$:

Where $C_1 = 0.0362$, $C_2 = 1.0937$, $C_3 = 25.724$

For $\gamma_{API} > 30$:

Where $C_1 = 0.0178$, $C_2 = 1.1870$, $C_3 = 23.931$

The corrected gas gravity accounts for separator conditions:

Applicability Range:

Glaso Correlation (1980)

Developed from North Sea crude oil samples. Particularly suitable for North Sea and similar light oils.

Where:

Applicability Range:

Al-Marhoun Correlation (1988)

Developed specifically for Middle East crude oils (Saudi Arabia, UAE, Kuwait).

Where $\gamma_o$ is oil specific gravity:

Applicability Range:

Petrosky-Farshad Correlation (1993)

Developed for Gulf of Mexico crude oils. Particularly accurate for deepwater GOM reservoirs.

Where:

Applicability Range:

Dindoruk-Christman Correlation (2001)

Developed for Gulf of Mexico crude oils with extended pressure range. Suitable for high-pressure reservoirs.

With coefficients:

• $a_1 = 0.0005121$
• $a_2 = 0.8403$
• $a_3 = 1.0891$
• $a_4 = 0.5187$
• $a_5 = 1.0000$
• $a_6 = 1.2321$

Applicability Range:

Correlation Selection Guide

By Geographic Region

By Oil Type

By Pressure Range

Functions Covered

See each function page for detailed parameter definitions, Excel syntax, and usage examples.

Applicability and Limitations

General Applicability

1. Valid only below or at bubble point: For $P \leq P_b$
2. Saturated oil condition: Gas is in solution with oil
3. Black oil assumption: No retrograde condensation

Limitations

1. Region-specific: Correlations developed for specific oil types may not transfer well to other regions.

2. Separator conditions: Vasquez-Beggs requires separator pressure and temperature for gas gravity correction. Other correlations assume standard separator conditions.

3. Extrapolation danger: Using correlations outside their development range can produce significant errors.

4. Volatile oils: Standard black oil correlations may underpredict $R_s$ for volatile oils with high GOR.

5. CO₂ content: High CO₂ content (common in some pre-salt fields) can significantly affect gas solubility. Most correlations do not account for this [2].

Comparison with Laboratory Data

When laboratory PVT data is available, compare with correlations:

1. Calculate AARE (Average Absolute Relative Error):

2. Acceptable error: AARE < 10-15% is generally acceptable for most engineering calculations

3. Bias check: Consistent over- or under-prediction suggests systematic error

Studies [2] have shown that for Brazilian pre-salt oils:

• Al-Shammasi (2001) showed best performance (AARE ~14%)
• Standing and Vasquez-Beggs also performed reasonably well

Related Topics

• Oil Formation Volume Factor (Bo) - Related to Rs
• Bubble Point Pressure (Pb) - Determines Rs validity range
• Oil Viscosity - Affected by dissolved gas
• PVT Overview - Correlation selection guide

References

1. Standing, M.B. (1947). "A Pressure-Volume-Temperature Correlation for Mixtures of California Oils and Gases." Drilling and Production Practice, API, 275-287.

2. Mangili, P.V. and Ahón, V.R.R. (2019). "Comparison of PVT Correlations for Predicting Crude Oil Properties: The Brazilian Campos Basin Case Study." Brazilian Journal of Petroleum and Gas, 13(3), 129-157.

3. Vasquez, M.E. and Beggs, H.D. (1980). "Correlations for Fluid Physical Property Prediction." Journal of Petroleum Technology, 32(6), 968-970. SPE-6719-PA.

4. Glaso, O. (1980). "Generalized Pressure-Volume-Temperature Correlations." Journal of Petroleum Technology, 32(5), 785-795. SPE-8016-PA.

5. Al-Marhoun, M.A. (1988). "PVT Correlations for Middle East Crude Oils." Journal of Petroleum Technology, 40(5), 650-666. SPE-13718-PA.

6. Petrosky, G.E. and Farshad, F.F. (1993). "Pressure-Volume-Temperature Correlations for Gulf of Mexico Crude Oils." SPE Reservoir Evaluation & Engineering, 1(5), 416-420. SPE-51395-PA.

7. Dindoruk, B. and Christman, P.G. (2004). "PVT Properties and Viscosity Correlations for Gulf of Mexico Oils." SPE Reservoir Evaluation & Engineering, 7(6), 427-437. SPE-89030-PA.